Carbon Concentration of Austenite in Austempered Ductile Iron (ADI)

Transcription

1 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G Published quarterly as the organ of the Foundry Commission of the Polish Academy of Sciences ISSN ( ) Volume 7 Issue 3/ /3 Carbon Concentration of Austenite in Austempered Ductile Iron (ADI) Z. Ławrynowicz *, S. Dymski Department of Materials Science and Engineering, Mechanical Engineering Faculty, University of Technology and Life Sciences, al. Kaliskiego 7, Bydgoszcz, Poland * lawry@utp.edu.pl Received on: ; Approved for printing on: Abstract The investigation was carried out to eamine the influence of temperature and times of austempering process on the maimum etend to which the bainite reaction can proceed and the carbon content in retained austenite. It should be noted that a small percentage change in the austenite carbon content can have a significant effect on the subsequent austempering reaction changing the volume of the phases present and hence, the resulting mechanical properties. Specimens were prepared from an unalloyed ductile cast iron, austenitised at 95 o C for 6 minutes and austempered by the conventional single-step austempering process at four temperatures between B S and M S, eg., 25, 3, 35 and 4 o C. The samples were austempered at these temperatures for 15, 3, 6, 12 and 24 minutes and finally quenched to ambient temperature. Volume s of retained austenite and carbon concentration in the residual austenite have been observed by using X-ray dif. Additionally, carbon concentration in the residual austenite was calculated using volume data of austenite and a model developed by Bhadeshia based on the McLellan and Dunn quasi-chemical thermodynamic model. The comparison of eperimental data with the T, T ' and Ae 3 ' phase boundaries suggests the likely mechanism of bainite reaction in cast iron is displacive rather than diffusional. The carbon concentration in retained austenite demonstrates that at the end of bainite reaction the microstructure must consist of not only ausferrite but additionally precipitated carbides. Keywords: Retained austenite, Bainite, Austempering, Ductile iron 1. Introduction The development of austempered ductile iron (ADI) is a major achievement in cast iron technology. The starting material for the development of ADI is the high quality ductile or nodular cast iron. It is then subjected to an isothermal heat treatment process known as austempering. The attractive properties of ADI are related to its unique microstructure that consists of ferrite and high carbon austenite. Because of this microstructure, the product of austempering reaction in ductile iron is often referred to as ausferrite rather than bainite. Large amount of silicon present in ductile iron suppresses the precipitation of carbide during austempering reaction and retains substantial amount of stable high carbon austenite. The austempering heat treatment enables the ductile cast iron containing mainly strong bainitic ferrite and ductile carbonenriched austenite, with some martensite transforms from austenite during cooling down to room temperature [1,2]. Fully bainitic austempered ductile irons are free from allotriomorphic ferrite and almost free from athermal martensite. The structure is thus composed of retained austenite, bainitic ferrite and carbides (Fig. 1). Carbides however, can be suppressed by alloying with elements such as Si and Al. As shown in Fig. 2 bainite growth by a diffusionless mechanism has to occur at a temperature just below T ** when the free energy of bainitic ferrite falls below that of austenite of the same composition. ** The T temperature can be defined [3] such that stress free austenite and ferrite of the same composition (with respect to both the interstitial and the substitutional alloying elements) are in metastable equilibrium. Thus any displacive transformation involving a full supersaturation of carbon (i.e. bainitic ferrite would then inherit the carbon content of the parent austenite) can occur only below the appropriate T temperature. Strain energy would have effect of shifting curve to lower carbon concentration, T curve [4]. A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,

2 Phase / % 1 5 Stage I Martensite Bainitic ferrite Heat treatment processing window Austempering time Carbide Stage II Ferrite Retained austenite log time Carbide transformation / % 1 Fig. 1. Schematic representation of the development of microstructure during austempering, together with an illustration of the processing window. Martensite is present only when the sample is cooled to room temperature before the austempering has been completed [1,2] Temperature Free energy, G T 1 T 1 Ae 1 T Ae 3 T Carbon concentration Fig. 2. Schematic illustration of the origin of the T curve on the phase diagram. and refer to ferrite and austenite, respectively. T 1 is the temperature corresponding to the free energy curves [3] At T 1 the bainitic reaction will cease when the carbon concentration of the residual austenite ( ) approaches the T curve: T (1) Then it is impossible for the displacive growth of bainite to proceed further. It is therefore possible to predict the carbon concentration in the retained austenite at the end of bainite transformation simply from chemical composition if the carbon to distribute evenly in the austenite. On the other hand, if bainite grows in a manner without supersaturation of ferrite, the reaction should be able to proceed until the carbon concentration in the retained austenite reaches the paraequilibrium Ae 3 ' phase boundary. During isothermal transformation the ecess carbon in the bainite partitions into the residual austenite, forcing the net plate to grow from carbon enriched austenite. The process finally ceases as the austenite carbon content reaches T value, leading to the so-called the incomplete reaction phenomenon [4]. This also eplains why the degree of transformation to bainite is zero at the bainite-start temperature (B S ) and increases with undercooling below B S. The purpose of the present paper is to demonstrate how a thermodynamic method can be used for solving a problem of the mechanism of bainite reaction in ADI and determination of the carbon concentration in the retained austenite. This should in principle enable to eamine the partitioning of carbon from supersaturated ferrite plates into adjacent austenite and the carbon content in retained austenite using analytical method. 2. Material and methods The chemical composition of the eperimental ductile iron is listed in Table 1. The concentration of alloying elements in the matri is obtained from the chemical analysis. Ductile iron blocks were produced in a commercial foundry furnace. The melt was poured into a standard Y block sands molds (ASTM A-395), which ensured sound castings. Specimens austenitised at T =95 o C for 6 minutes were rapidly transferred to a salt bath at austempering temperatures 25, 3, 35 and 4 o C, held for 15, 3, 6, 12 and 24 minutes and then water quenched to room temperature. After austenitisation at 95 o C the grain size is almost unchanged and austenite at the same time is saturated maimally by carbon. The microstructure of the as-cast material matri contains 4% ferrite and 6% pearlite, however graphite nodules in material is 11.5%. After heat treatment, the samples were prepared for metallographic analysis. The samples were etched using 2% nital. Optical micrographs were taken with a Nikon camera attached to a light microscope. The two-stage carbon replicas were eamined using a TESLA BS 54 transmission electron microscope operated at 8 kv. Hultgren [5], introduced the term paraequilibrium to describe the constrained equilibrium between two phases which are forced to have the same substitutional to iron atom ratio, but which achieve equilibrium with respect to carbon. Ferrite formation under conditions of paraequilibrium essentially implies that the substitutional lattice is configurationally frozen and transformation occurs at the rate controlled by the diffusion of carbon in the austenite, the driving force for paraequilibrium transformation is dissipated in this process alone. 94 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,

3 The matri carbon concentration is estimated analytical as [6]: = ( (%Mn ).11(%Si ).7(%Ni ) (%Cu ).3(%Mo ) where ) T + ( is the estimated carbon concentration of the austenite at the austenitising temperature T [7]. The M S temperature is a characteristic of the specific alloy composition and in particular the carbon concentration. The M S is generally higher when the residual austenite has lower carbon content. The martensite M S start temperatures was calculated using the method developed by Bhadeshia and it equals M S =117 o C [8]. The determined carbon concentration of residual austenite at the points where the formation of bainite terminate were compared against the etrapolated T, T ', and Ae 3 ' phase boundaries for austempered ductile iron. Table 1. Chemical composition of ductile cast iron ADI, wt-% C Si Mn P S Mg Cr Ni Mo The X-ray investigations were performed on the specimens heat treated after specific time of the isothermal bainite reaction at the given temperature. The total volume of the retained austenite was measured from the integral intensity of he (111) and (11) peaks. The presence of high silicon content in ADI retards the formation of cementite in ferrite an austenite. Then, the measurements of carbon concentration in retained austenite were carried out by using X-ray dif. The carbon concentration was calculated from measured lattice parameter of the retained austenite. The 2θ values for austenite peaks were used to calculate the d spacings with Bragg s law and then the lattice parameters. The lattice parameter of austenite (a ) is related to the known relationship between the parameter and the carbon concentration: a (nm)= (3) where is the carbon concentration in austenite, in weight %. The matri carbon concentration,, of the ductile iron was also determined eperimentally with Dron 1.5 diffractometer using Co K radiation on specimens austenitised at 95 C for 6 minutes and quenched to ambient temperature. It should be noted that a small percentage change in the austenite carbon content can have a significant effect on the subsequent austempering reaction changing the volume of the phases present and hence, the resulting mechanical properties. The definition of the martensite-start temperature is that it corresponds to the temperature where both nucleation and growth become diffusionless [8]. m ) T 2 + (2) It was found that after quenching from austenitising temperature 95 m C the calculated carbon content in matri is =1.44%C and measured carbon content is m =1.5%C, hence the obtained values are very close. Thus, the later value was taken for further calculation. 3. Results and discussion The calculated details for phase diagram of the studied ductile iron at selected temperatures of isothermal transformation after austenitising at 95 o C are presented in Table 2. Table 2. Calculated details for phase diagram Ti, o C mol To mol T o (+4 J/mol) mol Ae 3 - the paraequilibrium carbon concentration of austenite in mole (Ae 3 - ), - the T-zero carbon concentration in mole at the T T o curve, - the same but allowing for 4J/mol of stored energy in the ' T o ferrite Figure 3 shows a comparison of the measured carbon concentration of the austenite at the different stages of the transformation to bainite against the calculated concentrations based on the T, T ' and Ae 3 - conditions. It is usually assumed that the point where the microstructure of austempered ADI ceases to change represents full transformation. But in case of bainitic transformation, reaction ceases before the parent phase (austenite) has completely transformed. It means that at any temperature below B s and in the absence of any interfering secondary reactions only a limited quantity of bainitic ferrite forms before the reaction terminates. The determined carbon concentrations of the residual austenite at the different stages of the formation of bainite are compared with the T, T ' and Ae' 3 phase boundaries for investigated ADI in Figure 3. The diagram was calculated as in Ref. [9] using a model developed by Bhadeshia [9] based on the McLellan and Dunn quasi-chemical thermodynamic model [1]. The martensite reaction starts temperature M s is also marked on this diagram. The paraequilibrium phase boundary is chosen because A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,

4 Temperature, o C Temperature, o C T ' T "a" Ae 3 ' M S =117 o C,5,1,15, Carbon concentration in austenite, mole T ' T 24 min. 15 min. "a" 3 min. 6 min. 12 min. This makes more difficult for subsequent bainitic ferrite to grow, when the austenite becomes stabilised by increased carbon concentration. The maimum etent to which the bainite reaction can proceed is therefore determined by the composition of the residual austenite. A stage where diffusionless growth becomes thermodynamically impossible and the formation of bainitic ferrite terminates is where the carbon concentration of the austenite reaches the T ' or T curves. Thus, the incomplete reaction phenomenon supports the hypothesis that the growth of bainitic ferrite occurs without any diffusion with carbon being partitioned subsequently into the residual austenite. The reaction is said to be incomplete, since the austenite has not achieved its equilibrium composition (given by the Ae' 3 curve) at the point the reaction stops. If on the other hand, the ferrite grows with an equilibrium carbon concentration then the transformation should cease when the austenite carbon concentration reaches the Ae' 3 curve [4,11,12,13]. However, since the carbon concentration of the residual austenite when the reaction is found to stop is close to the T ' curve than the Ae' 3 boundary, the analysis suggest that bainite grows by displacive transformation, but carbon atoms partition into the residual austenite or precipitate as carbides shortly after growth is arrested. Similar results have previously obtained by Bhadeshia and Christian for other alloys [3,9]. Thus, it is found eperimentally that the transformation to bainite does indeed stop close to the T boundary (Fig. 3a and b). In Figure 3 the reaction seems to stop closer to the T line than to the T ' boundary. This might be eplained by the fact that the T ' line accounts for 4 J/mol of stored energy in the bainite. If this energy is reduced by plastic deformation of the surrounding austenite then a higher volume of bainite should be able to form. The thermodynamic restriction imposed by the T curve on the etent of bainite transformation can result in the formation of pools of retained austenite with a coarse, blocky morphology Fig. 4). However, austenite also appears in the form of thin films trapped in between bainitic ferrite plates Fig. 5). 25 2,3,4,5,6,7,8 Carbon concentration in austenite, mole Fig. 3. The calculated phase boundaries Ae' 3, T and T ' for the investigated ADI together with all the eperimental data of the measured carbon contents of the austenite. Enlarged of an area in the central part in Fig. 3 corresponds to the marked area a no substitutional alloying element partitioning occurs during bainite formation. In presented diagram the reaction is found to stop when the average carbon concentration of the residual austenite is closer to the T ' curve than the Ae' 3 boundary. The presented results can be eplained when it is assumed that bainitic ferrite grows without diffusion, but any ecess of carbon is soon afterwards rejected into the residual austenite by diffusion [11]. Struktura ziarn eutektycznych w Ŝeliwie sferoidalnym hartowanym z temperatury T = 95 o C; przemiana izotermiczna w temperaturze T pi = 4 o C i czasie τ pi = 12 min. Obszar graniczny. Dwustopniowa replika węglowa. Pow µ m Fig. 4. Carbon replica taken from sample austenitised at 95 o C and austempered at 4 o C for 12 min. This replica reveals the presence of carbides inside of bainite (arrowed) and shows large pools of blocky retained austenite 96 A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,

5 To V = (5) To C where V is volume of bainitic ferrite, is the average carbon concentration in the alloy, is the paraequilibrium carbon concentration in the bainitic ferrite (.3 wt. %), is To the carbon concentration of the austenite corresponding to the T curve, C is the amount of carbon, which is tied up as carbides (cementite). 1 µ m Fig. 5. Microstructure of ADI austenitised at 95 o C and austempered at 3 o C for 12 min. The carbon replica reveals the morphology of bainite which consists of fine plates of ferrite growing in clusters (arrowed) known as sheaves General features of the microstructure at the cell boundaries which are manganese enriched illustrated in Figs. 4 and 5 show how the morphology of bainitic ferrite changes with austempering temperature. Both upper and lower bainite consist of aggregates of plates of ferrite called sheaves. At lower austempering temperature (3 o C), the sheaves of bainite are finer (Fig. 5) because the driving force for transformation is higher as is the strength of the austenite [14]. As the austempering temperature increases the sheaves of bainite become thicker and so the laths are easier to resolve (Fig. 4). The carbon partitioning from bainite makes the untransformed austenite more stable and the austenite can be seen at the cell boundaries. It is observed from the micrographs that blocky regions of austenite are smaller at the lower austempering temperature. It is important to bear in mind that in general if more bainite is formed, less austenite is available for retention because bainite formation consumes austenite. Moreover, in the residual austenite after transformation to bainite is inhomogeneous distribution of carbon. The austenite is enriched to a greater etent in the immediate vicinity to bainite platelets or in the region trapped between the platelets, while other region contains relatively poor carbon [11]. The above effect can be eaggerated in ADI, since cast iron is usually etremely segregated. When the matri of ADI only consist of ausferrite, thus: V + V =1 (4) and the permitted of bainite (V ) can be determined from Lever rule applied to the T curve, ( Fig. 6). The maimum volume of retained austenite (V ) will then equal 1- V. Coarse carbides were not observed but it seems from the micrographs that more fine dispersed particles of carbides can be seen inside sheaves of bainite (Figs. 4 and 5, arrowed). In case of carbides precipitation the maimum volume of bainitic ferrite (V ) can be calculated using the following equation [15]: Temperature V = T T T T Carbon concentration Fig. 6. Application of the lever-rule to the T curve allows the estimation of the permitted of bainite (V ) at any temperature (where for 95 o C =1,5 wt.% C =,3%C) Thus, the maimum volume of bainite taking into account cementite precipitation can be calculated using the relationship (5). It is seen in Fig. 7 that precipitation of cementite leads to an increase of volume of bainitic ferrite. Carbides locally reduce the carbon content of the parent austenite and increase the driving force for further ferrite growth. Cementite can precipitate from bainitic ferrite and also from supersaturated austenite. The growth of bainite is probably diffusionless but any ecess carbon in the supersaturated ferrite soon afterwards partitions into the residual austenite or precipitates within bainitic ferrite in the form of carbides [15,16]. When the process of carbon partitioning into the residual austenite is rapid relative to that of carbide precipitation, the transformation product is called upper bainite, whereas lower bainite is obtained when some of the carbon supersaturation is relieved by precipitation within the bainitic ferrite [14]. Austenite is supersaturated with respect to cementite θ θ precipitation when >, where denotes carbon concentration at the etrapolated A cm boundary. This means for the bainite reaction, that ferrite stops when T θ > since the growth of bainitic reaches the value T given by the T curve of the phase diagram. A consequence of the precipitation of cementite from austenite is that its carbon concentration drops A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,

6 below T, so that the growth of bainitic ferrite can continue to an etent larger than would be otherwise possible (see Fig. 7). Volume of bainite 1,8,6,4,2 12% 11% 1% % Fe 3 C Austempering Temperature, o C 15% Fe 3 C 14% Fe 3 C 13% Fe 3 C Fig. 7. Calculated the maimum volume of bainite in investigated ADI taking into account cementite precipitation in the range from % to 15 wt % Fe 3 C Conclusions 1. The amount of bainite that forms increases with time of austempering and as the transformation temperature is reduced below the B S temperature. 2. The etent of transformation to bainite in ductile iron, as in steels, decreases when increasing the isothermal transformation temperature towards the bainite start temperature. This is because the austenite can only transform to bainite if its carbon concentration is less than a value To given by the T curve. 3. The carbon concentration of the residual austenite increases during bainitic transformation as a consequence of the increasing volume of bainitic ferrite. 4. Transformation ceases before the austenite achieves its equilibrium composition given by Ae' 3 boundary, so that the effect is called the "incomplete-reaction phenomenon". The T restriction means that equilibrium, when the austenite has a composition given by the Ae 3 phase boundary, can never be reached, as observed eperimentally. 5. The ecess carbon in the bainitic ferrite partitions into the residual austenite and partitions inside the sheaves of banitic ferrite. 6. Precipitation of cementite leads to an increase of volume of bainitic ferrite. Carbides locally reduce the carbon content of the parent austenite and increase the driving force for further ferrite growth. 7. The bainite transformation in cast iron is essentially identical to that in steel. In steel, it has been demonstrated that the carbon concentration of the residual austenite reaches the critical value represented by the T curve will render the displacive bainite reaction to cease. Therefore the carbon concentration of austenite can be estimated by the thermodynamics principles described here. Since cast iron is etremely segregated, determined by X-ray dif is richer than that corresponds to the T curve in ADI. References [1] M.A. Yescas, H.K.D.H. Bhadeshia, D.J. Mac Kay, Estimation of the amount of retained austenite in austempered ductile irons using neural networks, Materials Science and Engineering, A311 (21) [2] Z. Ławrynowicz, S. Dymski, Application of the mechanism of bainite transformation to modelling of processing window in ductile iron ADI, Archives of Foundry Engineering, PAN, Vol.6, No 19, (26) (in Polish). [3] J.W. Christian, Theory of transformations in metals and alloys, 778, Oford, Pergamon Press, [4] H.K.D.H. Bhadeshia, Bainite in Steels, Institute of Materials, 1-458, London, [5] A. Hultgren, Jernkontores Ann., vol. 135 (1951) 43 [6] S.H. Zahiri at al., Application of bainite transformation model to estimation of processing window boundaries for Mn-Mo-Cu austempered ductile iron, Materials Science and Technology Vol.17 (21) [7] Z. Ławrynowicz, S. Dymski, Mechanism of bainite transformation in ductile iron ADI, Archives of Foundry Engineering, PAN, Vol.6, No 19, (26) (in Polish). [8] H.K.D.H. Bhadeshia, Bainite: Overall Transformation Kinetics, Journal de Phys., Vol. 43 (1982) C [9] H.K.D.H. Bhadeshia, A Rationalisation of Shear Transf. in Steels, Acta Metall., Vol.29 (1981) [1] R.B. McLellan, W.W. Dunn, J. Phys. Chem. Solids. Vol.3 (1969) [11] Z. Ławrynowicz, Materials Science and Technollogy, Vol 18 (22) [12] H.K.D.H. Bhadeshia, Diffusion of carbon in austenite, Metal Science, Vol.15 (1981) [13] Z. Ławrynowicz, A. Barbacki, The mechanism of bainite transformation in Fe-Cr-Mn-Si-C steel. Proc. of the Scientific Con. AMTECH 95, Rousse, Bułgaria, April 1995, 1-8 [14] Z. Ławrynowicz, Transition from upper to lower bainite in Fe-Cr-C steel, Mat. Sci. Techn., Vol.2 (24) [15] M. Takahashi, H.K.D.H. Bhadeshia, A Model for the Microstructure of Some Advanced Bainitic Steels, Materials Transaction, JIM, Vol.32 (1991) [16] R.C. Thomson at al., Modelling microstructural evolution and mechanical properties of austempered ductile iron, Materials Sci. and Technology, Vol. 16 (2) A R C H I V E S o f F O U N D R Y E N G I N E E R I N G V o l u m e 7, I s s u e 3 / 2 7,